This invention relates to a fuel supply control method for an internal combustion engine at deceleration, and more particularly to a method of this kind which is adapted to make it possible to accurately determine a fuel cut effecting region of the engine at deceleration of the engine, while the engine is operating in a low engine speed region as well as a high engine speed region, to thereby improve the emission characteristics, fuel consumption, etc. of the engine.
When the intake passage pressure is low, at deceleration of the engine with the throttle valve fully closed, if supply of fuel to the engine is carried out, a large quantity of unburnt fuel is emitted along with the exhaust gases, thereby badly affecting the fuel consumption, emission characteristics, etc. of the engine. Also, in an internal combustion engine having a device for purifying the exhaust gases, like a three-way catalyst, a large quantity of unburnt fuel emitted together with the exhaust gases can cause burning of the three-way catalyst bed, thereby increasing the emission of detrimental exhaust gases. A method for preventing the inconveniences such as described above is already known, which carries out fuel cut while the engine is operating in a predetermined operating region at deceleration.
According to this known method, when the engine rpm is high, if the determination as to whether or not the engine is operating in the above predetermined operating region is made on the basis of the throttle valve opening, it can happen that fuel cut is not carried out even when the intake passage absolute pressure is low enough for fuel cut to be carried out, resulting in the above-mentioned inconveniences. Therefore, it has previously been proposed by the assignee of the present application in Japanese Provisional Patent Publication No. 57-191426 that when the engine rpm is high, the above predetermined operating region be determined on the basis of the intake passage absolute pressure as well as the engine rpm, even if the throttle valve is not fully closed.
However, even when the engine rpm is low, if the determination as to whether or not the engine is operating in the above predetermined operating region is made on the basis of the engine rpm and the intake passage absolute pressure, it becomes difficult to discriminate between the fuel cut operating region in which the throttle valve is almost fully closed and a low load operating region in which the throttle valve is slightly opened (e.g. the throttle valve opening is 10.degree.). For example, if the determination that the conditions for fuel cut stand is made on the basis of a certain value of the intake passage absolute pressure which is a little higher than the intake passage absolute pressure occurring when the engine is idling with the throttle valve fully closed, supply of fuel to the engine can be interrupted contrary to the intention of the driver to continue to operate the engine in the above low load operating condition. According to this manner of determination, therefore, it becomes impossible to achieve a certain engine operating condition desired by the driver, resulting in remarkable deterioration of the driveability of the engine. Moreover, it becomes the more difficult to determine with accuracy the above predetermined operating region because the intake passage absolute pressure varies with changes in atmospheric pressure while the throttle valve is fully closed.
Further, it has also been disclosed in Japanese Provisional Patent Publication No. 57-191426, referred to above, that the predetermined value of the engine rpm and the predetermined value of the intake passage absolute pressure employed for determining whether or not the engine is operating in the above predetermined operating region are each set to values different between at fuel cut initiation and at fuel cut termination, that is, to provide a hysteresis characteristic so as to improve the driveability of the engine. Due to this hysteresis characteristic, the phenomenon can be prevented that alternate fuel cut initiation and fuel cut termination repeatedly take place when the value of either of the engine parameters, i.e. engine rpm, intake passage absolute pressure for determining the fuel cut effecting region fluctuates across a predetermined fuel cut determining value to thereby deteriorate the driveability of the engine.
The value of the difference between a predetermined value of intake passage absolute pressure for determining the fuel cut initiation condition and another predetermined value of same for determining the fuel cut termination condition, that is, the hysteresis margin has hitherto been set only in order to compensate for errors in the value outputted by the intake passage absolute pressure sensor and fluctuations in the engine operating parameters which are estimated empirically and experimentally. However, the value of the above difference should be set by also taking into account the phenomenon that usually the value of intake passage absolute pressure while the engine is operating in a fuel cut or non-combustion operating condition (hereinafter merely called "the motoring condition") is higher than the value of intake passage pressure while the engine is operating in a normal combustion operating condition. That is, the actual intake air quantity supplied to the engine while the engine is operating in a normal combustion operating condition is larger than the intake air quantity when the engine is in the motoring condition, so far as the engine rpm remains constant, or in other words, the charging efficiency of the engine in a normal combustion operating condition is higher than that when the engine is in motoring condition, as generally well known. This means that the intake passage absolute pressure during the motoring condition is higher than that during normal combustion operating condition of the engine, so far as the intake air quantity remains constant. This phenomenon will cause a heavy deterioration in the driveability of the engine when the engine is operating in an operating condition very close to a predetermined fuel cut effecting region. More specifically, if fuel cut is carried out consequently upon the intake passage absolute pressure becoming lower than a predetermined fuel cut determining value, while the engine is slowly decelerating, the intake passage absolute pressure during the motoring operation of the engine increases to a value higher than that assumed when the engine is in a normal combustion operating condition. When the intake passage absolute pressure during the motoring operation becomes higher than the above predetermined fuel cut determining value, fuel cut is terminated on the judgement that the engine is no longer operating in the fuel cut effecting region. Thereafter, if fuel supply to the engine is resumed so as to operate the engine in normal combustion operating condition, the intake passage pressure drops to initiate fuel cut again. This phenomenon repeatedly occurs, resulting in remarkable deterioration of the driveability of the engine.
Further, it is desirable that the fuel cut operation should be continued until the engine decelerates to as low rpm as possible so as not to spoil the emission characteristics and fuel consumption of the engine. However, when the engine is in a cold condition, the sliding parts of the engine have high frictional resistance to make the operation of the engine unstable when the engine speed is very low. Therefore, if the engine rotational speed at which the fuel cut operation is to be terminated is set at too low rpm, engine stall can easily occur after termination of the fuel cut operation, for instance, upon disengagement of the clutch of the engine.